Photochromic articles have been prepared by incorporating photochromic materials within the article. This has been accomplished by blending photochromic material within one or more precursors of the article, as for example by incorporating a photochromic material within a polymerizable composition used to prepare the article. Another approach that has been suggested is to imbibe photochromic material into and below the surface of the article by conventional imbibition techniques, e.g., by thermal diffusion. Such articles are reported to have sufficient free volume within the article to allow photochromic material, e.g., organic photochromic material, to physically transform from what is usually a colorless form to a colored form when exposed to actinic radiation, and then revert to the original colorless form when the actinic radiation is removed.
There are however materials that are not susceptible to the foregoing methods. These materials are reported to have insufficient free volume within the matrix or body of the material, e.g., within the region just below the surface of the material, to allow their use for commercial photochromic applications. Such materials include conventional glass (in connection with organic photochromic materials); thermoset polymers, such as those prepared from compositions comprising polyol (allyl carbonate) monomers, notably allyl diglycol carbonate monomers, e.g., diethylene glycol bis(allyl carbonate), and copolymers thereof; thermoplastic polymers having a high glass transition temperature, e.g., the commonly known thermoplastic bisphenol A-based polycarbonates; highly cross-linked optical polymers; and other such polymer materials. In order to allow the use of such materials for photochromic applications, it has been proposed to apply photochromic coatings, e.g., organic coatings, to their surface.
For some applications it is economically attractive to use photochromic-containing, radiation-curable organic coating compositions. These coating compositions are applied to the surface of the chosen non-photochromic receptive material and cured by, for example, exposure to ultraviolet light. Typically, radiation-curable coating compositions contain a photoinitiator to initiate the curing mechanism. Generally, photoinitiator compounds have an aromatic ring in their structure, which effectively absorbs ultraviolet light. Moreover, they are usually of low molecular weight to improve their solubility in the radiation-curable composition, and consequently are relatively volatile when subjected to heat. These properties can cause yellowing of the cured coating and produce unpleasant odors respectively when the curable and cured coating composition containing the photoinitiator is subjected to heat and light during and after curing. Further, it is known that unreacted photoinitiators remain in the cured coating composition after curing, and can be exuded from the coating.
It is therefore desirable to utilize photochromic-containing coating compositions, e.g., radiation-curable coating compositions, which do not require a photoinitiator for curing, or require a lower amount of photoinitiator than is generally used to cure radiation-curable coating compositions. Moreover, it would be desirable to utilize such coating compositions as photochromic coatings for materials that are not receptive to incorporating photochromic materials within the matrix (core) or subsurface region of the material.